Second-order cybernetics (also known as the cybernetics of cybernetics, and the New cybernetics ) was developed between 1968 and 1975 in recognition of the power and consequences of cybernetic examinations of circularity. It is cybernetics, when cybernetics is subjected to the critique and the understandings of cybernetics. It is the cybernetics in which the role of the observer is appreciated and acknowledged rather than disguised, as had become traditional in western science: and is thus the cybernetics that considers observing, rather than observed systems. In this article, the rationale from and through the application of which, second-order cybernetics was developed is explored, together with the contributions of the main precursors and protagonists. This is developed from an examination of the nature of feedback and the Black Box both seen as circular systems, where the circularity is taken seriously. The necessary presence of the observer doing the observing is established. The primacy of, for example, conversation over coding as a means of communication is arguedone example of circularity and interactivity in second-order cybernetic systems. Thus second-order cybernetics, understood as proposing an epistemology and (through autopoietic systems) an ontogenesis, is seen as connected to the philosophical position of Constructivism. Examples are given of the application of second-order cybernetics concepts in practice in studies of, and applications in, communication, society, learning and cognition, math and computation, management, and design. It is asserted that the relationship between theory and practice is not essentially one of application: rather they strengthen each other by building on each other in a circularity of their own: the presentation of one before the other results from the process of explanation rather than a necessary, structural dependency. Finally, the future of second-order cybernetics (and of cybernetics in general) is considered. The possibility of escalation from second to third and further orders is considered, as is the notion that second-order cybernetics is, effectively, a conscience for cybernetics. And the popular use of “cyber-” as a prefix is discussed.